A) Field of the Invention
This invention relates to a charge transfer device and a solid-state imaging device using the charge transfer device, and more specifically to an overflow drain structure of the charge transfer device.
B) Description of the Related Art
Conventionally, in a solid-state imaging device using a charge transfer device, for example, a signal charge of an arbitral vertical line of a photoelectric conversion element is thinning out alternatively by providing an overflow drain for draining a signal charge to a vertical charge transfer device. (For example, refer to Japanese Laid-Open Patent Hei6-338524.)
FIGS. 7 are drawings showing a charge-discharging structure in the conventional charge transfer device of the solid-state imaging device. FIG. 7A is a plan view showing a charge-discharging structure in the conventional charge transfer device of the solid-state imaging device.
A solid-state imaging device 300 is consisted of a multiplicity of photoelectric conversion elements 381 arranged in a tetragonal matrix, plurality of columns of vertical charge transfer devices (VCCD) 382, a horizontal charge transfer device (HCCD) 383 and an output circuit 384.
A signal charge 387 stored in the photoelectric conversion elements 381 is vertically transferred from the upper side to the lower side in the drawing by the adjacent vertical charge transfer device 382. The horizontal charge transfer device 383 receives the signal charges 387 transferred by plurality of columns of the vertical charge transfer devices 382 in parallel and transfers them to the output circuit 384 one after another. The output circuit 384 outputs the signal charges 387 transferred by the horizontal charge transfer device 383 to outside of the solid-state imaging device 300.
A charge-discharging device 390 is formed around the horizontal charge transfer device 383 near the lower end of the vertical charge transfer device 382. The charge-discharging device 390 is consisted of a transfer circuit 391, discharging control gate 393 and a overflow drain 395 and can discharge the signal charge 387 transferred by the vertical charge transfer device 382 to outside of the solid-state imaging device 300.
FIG. 7B is a schematic cross sectional view showing a structure of the charge-discharging device 390.
The transfer circuit 391 is consisted of n-type transfer channel (hereinafter called transfer channel) 391c formed on the surface of p-well (or p-type substrate) 385, and transfer electrode 391e formed above transfer channel 391c with the insulating film 386 therebetween, and forms one transfer unit of the vertical charge transfer device 382. A transfer voltage supplying line 392 supplies a control voltage φvn to the transfer electrode 391e. 
The discharging control gate 393 is consisted of a transfer channel 393c which is an area between the n-type circuit formed as overflow drain 395 and the transfer channel 391c of the transfer circuit 391, and a discharging control gate electrode 393e formed above discharging channel 393c with the insulated film 386 therebetween. Turning on/off of the discharging control gate 393 is controlled by control voltage φrc supplied by the discharging control voltage supply line 394. Moreover, when the control voltage φrc is in a state of high level, the discharging control gate is ON, and when the control voltage φrc is in a state of low level, the discharging control gate is OFF.
The overflow drain 395 is consisted of an n-type area formed on a surface of the p-well (or p-type substrate) 385 and is a drain for discharging the signal charge 387 to the outside. The drain voltage supplying line 396 supplies a drain voltage Vdr to the drain 395.
FIG. 7C is an electrical potential distribution map formed in a semiconductor of the charge-discharging device 390 shown in FIG. 7B.
Electrical potential 397 shows channel electrical potential of the transfer channel, electrical potential 398off shows channel electrical potential when the drain operation is turned off (control voltage φrc is in the state of low level), electrical potential 398on shows channel electrical potential when the drain operation is turned on (control voltage φrc is in the state of high level), and electrical potential 399 shows drain electrical potential of the voltage overflow drain 395.
During the solid-state imaging elements 300 is being operated normally, the charge-discharging control electrode 393e maintains the state of turned-off (control voltage φrc is being at the low level), and the signal charge 387 transferred in the vertical charge transfer device 382 is not discharged to the outside, but is transferred to the horizontal charge transfer device 383. Then, depending on necessity, when the signal charge 387 is transferred to the transfer channel 391c, as shown with an dotted arrow in the drawing, by turning on the charge-discharging control electrode 393e (making the control voltage φrc at the high level), the signal charge 387 can be drained from the transfer channel 391c to the charge overflow drain 395 via the discharging channel 393c. 
According to the above-described operation, since it is operated at once in plurality of the electric charge-discharging device 390 arranged in parallel, the signal charge of the one horizontal line of the photoelectric conversion element 381 that was chosen can be alternatively thinned out by changing on-off of electric charge drain control electrode 393e at specific timing.
Generally, there may be a potential barrier as shown in FIG. 7C at a certain probability in the transfer channel 391c, for example, by manufacturing unevenness. When there is a potential barrier 389, the electric charge below a fixed amount cannot be drained by the charge-discharging device 395. In the above-described electric charge-discharging device 390, the signal electric charge 387 may be remained by the electric potential barrier 389 in the transfer channel 391c having the electric potential barrier 389 when the signal electric charge 387 is drained to the charge-discharging device 395 with the electric charge-discharging control electrode 393e turned on. The remained signal electric charge is output from the vertical charge transfer circuit 382 through the horizontal charge transfer device 383 after the drain operation finishes.
For example, all the signal electric charges are drained to the charge-discharging device 395 by the electric charge-discharging device 390, the remained electric charge is output from the vertical line having the electric potential barrier 389, and it appears as a white line on a reproduced screen. This phenomenon will appear as a picture superimposed by the white line on a digital still camera etc. also in a case of the well-known process for thinning out one-half of the vertical scanning lines, and will worsen quality of image remarkably.